A. Field of the Invention
The present invention relates to wood flake molding.
B. Background of the Art
Wood flake molding, also referred to as wood strand molding, is a technique invented by wood scientists at Michigan Technological University during the latter part of the 1970s for molding three-dimensionally configured objects out of binder coated wood flakes having an average length of about 1xc2xc to about 6 inches, preferably about 2 to about 3 inches; an average thickness of about 0.005 to about 0.075 inches, preferably about 0.015 to about 0.030 inches; and an average width of 3 inches or less, most typically 0.25 to 1.0 inches, and never greater than the average length of the flakes. These flakes are sometimes referred to in the art as xe2x80x9cwood strands.xe2x80x9d This technology is not to be confused with oriented strand board technology (see e.g., U.S. Pat. No. 3,164,511 to Elmendorf) wherein binder coated flakes or strands of wood are pressed into planar objects. In wood flake or wood strand molding, the flakes are molded into three-dimensional, i.e., non-planar, configurations.
In wood flake molding, flakes of wood having the dimensions outlined above are coated with MDI or similar binder and deposited onto a metal tray having one open side, in a loosely felted mat, to a thickness eight or nine times the desired thickness of the final part. The loosely felted mat is then covered with another metal tray, and the covered metal tray is used to carry the mat to a mold. (The terms xe2x80x9cmoldxe2x80x9d and xe2x80x9cdiexe2x80x9d, as well as xe2x80x9cmold diexe2x80x9d, are sometimes used interchangeably herein, reflecting the fact that xe2x80x9cdiesxe2x80x9d are usually associated with stamping, and xe2x80x9cmoldsxe2x80x9d are associated with plastic molding, and molding of wood strands does not fit into either category.) The top metal tray is removed, and the bottom-metal tray is then slid out from underneath the mat, to leave the loosely felted mat in position on the bottom half of the mold. The top half of the mold is then used to press the mat into the bottom half of the mold at a pressure of approximately 600 psi, and at an elevated temperature, to xe2x80x9csetxe2x80x9d (polymerize) the MDI binder, and to compress and adhere the compressed wood flakes into a final three-dimensional molded part. The excess perimeter of the loosely felted mat, that is, the portion extending beyond the mold cavity perimeter, is pinched off where the part defining the perimeter of the upper mold engages the part defining perimeter of the lower mold cavity. This is sometimes referred to as the pinch trim edge.
U.S. Pat. Nos. 4,440,708 and 4,469,216 disclose this technology. The drawings in U.S. Pat. No. 4,469,216 best illustrate the manner in which the wood flakes are deposited to form a loosely felted mat, though the metal trays are not shown. By loosely felted, it is meant that the wood flakes are simply lying one on top of the other in overlapping and weaving fashion, without being bound together in any way. The binder coating is quite dry to the touch, such that there is no stickiness or adherence which holds them together in the loosely felted mat. The drawings of U.S. Pat. No. 4,440,708 best illustrate the manner in which a loosely felted mat is compressed by the mold halves into a three-dimensionally configured article (see FIGS. 2-7, for example).
This is a different molding process as compared to a molding process one typically thinks of, in which some type of molten, semi-molten or other liquid material flows into and around mold parts. Wood flakes are not molten, are not contained in any type of molten or liquid carrier, and do not xe2x80x9cflowxe2x80x9d in any ordinary sense of the word. Hence, those of ordinary skill in the art do not equate wood flake or wood strand molding with conventional molding techniques.
One limitation heretofore associated with this technology has been forming holes in molded wood strand parts. The part tends to be too weak at the perimeter of the hole.
In the present invention it has been discovered that narrowing the space between the top and bottom molds in the area immediately surrounding and defining a molded hole can strengthen the hole perimeters. By compressing a relatively uniformly thick mat into a narrower space in the perimeter of the hole, a denser, thinner, and stronger perimeter around the hole is created. Furthermore, where the hole is in a raised boss, the entire boss can be strengthened and densified in a similar manner.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification and claims.